In electrophotographic copiers, an image formed on the photosensitive drum is transferred to the copy paper by means of a transfer charger. Thereafter, the paper is separated from the drum by means of a well known corona charger used as a separation device. Japanese Laid-Open Patent Application SHO No. 58-120282 discloses a scorotron charger used as a separation charger, the scorotron charger having a grid electrode interposed between a corona wire and the object to be charged, and which controls the amount of charge by controlling the voltage applied to the grid.
In general, paper transported via a fixing device of a copying apparatus is somewhat curled. Particularly, paper tends to curl more greatly at both its ends than at its central portion in a direction perpendicular to the paper transport direction. When the paper is re-fed with both its ends curling towards a photosensitive member in a duplex or a composite copy mode, an excellent separability cannot be obtained at the curling portion. As a result, a separation charger is operated to discharge an excessive amount of charge for completely separating the paper. Accordingly, toner once deposited onto the paper is again transferred to the photosensitive member, producing a phenomenon in which a part of an image on the copy paper is lost (This phenomenon is hereinafter referred to as a "mark").